GaN devices typically include a barrier region, such as AlGaN or InAlGaN, formed on a GaN buffer to automatically form a two-dimensional (2D) charge carrier gas channel near the barrier-buffer interface. The channel is formed by a 2D electron gas for n-channel devices and by a 2D hole gas for p-channel devices. In the fabrication of such GaN devices, a shift in the threshold voltage of the transistors can occur for various reasons. A threshold voltage shift results in an increase in the variation of the threshold voltage across the wafer, reducing yield.
One cause of threshold voltage shift in GaN devices is the so-called charging of the silicon nitride (SiN) layer typically formed on the surface of the barrier region in order to prevent oxidation of the barrier surface. Charging involves the storage and generation of charge traps in the silicon nitride layer. These charge traps shift the device threshold voltage when the silicon nitride layer is interposed between the active semiconductor region and certain metallizations, such as the gate structure. Electron and hole traps can arise in silicon nitride at point defects such as paramagnetic defects (e.g. ≡Si., ≡Si—O—O., ≡Si2N.), diamagnetic defects (e.g. ≡Si—Si≡, ═N—H), two coordinated Si-atoms with lone pair electrons (e.g. ═Si:), neutral defects (e.g. ≡SiO., ≡SiOH), charged defects (e.g. ≡Si., +Si≡), intrinsic defects (e.g. ≡Si., ═N—N═, ≡Si—O—O—Si≡) and extrinsic defects (e.g. ≡SiH, ≡Si2NH, ≡SiOH). Electron and hole traps can also arise in silicon nitride in the presence of existing residual Si—H bonds (so-called silicon-hydrogen-complexes), due to the polarity of the Si—H binding. The formation of silicon nitride produces residual hydrogen in the silicon nitride, forming the Si—H bonds, which have a localized hydrogen valence electron in a covalent bond between a hydrogen and silicon atom. Si—H bonds have a relatively deep energy trap level and, therefore, the resulting charge traps are stable at room temperature.
In each case, the storage and generation of holes and electrons in a silicon nitride layer formed on the surface of a barrier region of a GaN device result in charging of the silicon nitride layer. Various standard GaN processing steps cause such charging. This charging is caused in part by ultraviolet (UV) light which is present in standard GaN processing. UV light impinging on a GaN device during operation also causes charging of the silicon nitride anti-oxidation layer, meaning that the threshold voltage variation can change over time. In general radiation with higher energy than the bandgap energy of silicon nitride can degrade GaN device performance and reliability.